Brexit is going to be a big test for the union that ended 'centuries of repeated wars in Europe'

European Commission President Jean-Claude Juncker attends a news conference at the Delegation of the European Union to China in Beijing, July 13, 2016. REUTERS/Thomas Peter LONDON - The German government warned about threats to 70 years of political integration that ended "repeated wars," ahead of a meeting of the 28 leader of the European Union nations.

And did not shy away from strong language to highlight the EU's role in keeping the peace.

"Our top priority is the cohesion and unity of the EU-27 and to safeguard what has been achieved in the common legal and political framework for the future of the EU at 27. European integration has brought peace and prosperity for the past 70 years after centuries of repeated wars in Europe," the memo said (emphasis ours).

The man charged with keeping the European front united in the negotiations is former internal markets commissioner Michel Barnier. The German memo agreed with his position that detailed trade talks should only begin once a multi-billion euro bill to exit is agreed upon, as well as the future of European citizens living in the UK.

"First of all, elements of the process of British EU membership and exit will have to be settled," the memo reported by Bloomberg said. "Only then will it be possible to negotiate all the political, legal and economic questions regarding the future relationship."

While Brexit is a key threat to European unity, it's not the only one. Germany is coming under fire for its powerful role as the region's biggest economy and most influential consensus builder.

Poland is becoming increasingly isolated in the EU and, on Thursday, vehemently objected to the EU reappointing Donald Tusk as President of the European Council.

Tusk, the former prime minister of Poland, has criticised the current government's policies, putting him at odds with his home nation, while the Polish government aimed its anger at Germany for the vote to keep him as President.